.gb is an Internet country code top-level domain (ccTLD) for the United Kingdom. Introduced at the same time as the UK's other top-level domain (.uk), it was never widely used, and has since fallen into disuse.
The normal rule in the Domain Name System is that a top-level domain for a country is derived from the corresponding two-letter code in the ISO 3166-1 list. According to that list the code for the United Kingdom of Great Britain and Northern Ireland is GB, and the .gb top-level domain was therefore delegated. However, GB is a common abbreviation for Great Britain, which is not the same as the United Kingdom (Great Britain does not include Northern Ireland). At the time only a very limited number of countries were on the Internet (fewer than 6), so the UK applied to Jon Postel of IANA for .uk. This was granted, and all UK domain names are now issued as .uk not .gb.
.gb was actually used for a number of years. The only major site to use it was DERA at their address dra.hmg.gb. (HMG standing for Her Majesty's Government.) It is now unused.
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